Surface protection method of sintered NdFeB magnets and products thereof

Abstract

The present invention relates to a surface protection method for a sintered NdFeB magnet and a product thereof. The method comprises: providing a sintered NdFeB magnet; forming a zinc-aluminum-titanium protective liquid on the surface of the sintered NdFeB magnet to obtain a preform, wherein the zinc-aluminum-titanium protective liquid mainly comprises metal powder, molybdate and silane coupling agent, and the metal powder comprises titanium powder, flaky zinc powder and flaky aluminum powder, wherein the average sheet thickness of the flaky zinc powder and the flaky aluminum powder is less than or equal to 2.5 μm, and the thickness size deviation is less than or equal to 25%, the average sheet diameter of the flaky zinc powder and the flaky aluminum powder is greater than or equal to 15 μm, and the diameter size deviation is less than or equal to 30%, the particle size of the titanium powder is 1.5 μm-5 μm, and the mass fraction of the titanium powder in the metal powder is 0.8%-15%; drying the preform and sintering it at 250°C-370°C to form a protective layer on the surface of the sintered NdFeB magnet. The method can improve the protective performance of the protective layer on the basis of maintaining the high magnetic properties of the sintered NdFeB magnet.

Description

Technical Field

[0001] The invention relates to the field of magnetic material surface protection, in particular to a surface protection method for a sintered NdFeB magnet and a product thereof. Background Art

[0002] Sintered NdFeB permanent magnet materials are widely used due to their high magnetic properties. However, the corrosion resistance of sintered NdFeB in various environments is poor. Increasing surface protection treatment is the main method to improve the corrosion resistance of NdFeB magnets. At present, the surface protection treatment technologies used for sintered NdFeB permanent magnet materials include composite electroplating, composite chemical plating, hot dip plating, electrophoretic coating, magnetron sputtering, etc. Although the corrosion resistance of sintered NdFeB permanent magnet materials can be improved to varying degrees through surface protection treatment, the surface protection treatment process will cause performance damage to the sintered NdFeB permanent magnet materials, and there are also unsolvable environmental pollution problems such as air and water. Therefore, in the field of sintered NdFeB permanent magnet materials, there is an increasingly urgent need to develop environmentally friendly protective coating technology with high corrosion resistance.

[0003] Dacromet coating has excellent corrosion resistance and can be widely used in marine engineering, shipbuilding, power chemical industry and other fields. However, the traditional Dacromet coating contains heavy metal chromium ions (Cr 6+ ) and is restricted for use. As a result, chromium-free zinc-aluminum (Zn-Al) coating technology that meets new environmental protection requirements came into being and became a new technology in the field of steel surface protection. However, due to the low corrosion resistance of sintered NdFeB permanent magnet materials, the problem of magnet corrosion still occurs during the process of coating the Zn-Al coating on the surface of the sintered NdFeB permanent magnet material, thereby affecting the magnetic properties of the magnet. In addition, the Zn-Al coating also has problems such as weak bonding with the magnet and poor protection performance. Summary of the invention

[0004] Based on this, it is necessary to provide a surface protection method for sintered NdFeB magnets and its products to address the above problems; the surface protection method forms a protective layer on the surface of the sintered NdFeB magnet, which can significantly improve the protective performance of the protective layer while maintaining the high magnetic properties of the sintered NdFeB magnet, and enhance the bonding force between the protective layer and the magnet.

[0005] A surface protection method for a sintered NdFeB magnet comprises the following steps:

[0006] Provide sintered NdFeB magnets;

[0007] A zinc-aluminum-titanium protective liquid is formed on the surface of the sintered NdFeB magnet to obtain a preform; wherein the zinc-aluminum-titanium protective liquid mainly comprises metal powder, molybdate and silane coupling agent, the metal powder comprises titanium powder, flaky zinc powder and flaky aluminum powder, wherein the average sheet thickness of the flaky zinc powder and the flaky aluminum powder is less than or equal to 2.5 μm, and the thickness size deviation is less than or equal to 25%, the average sheet diameter of the flaky zinc powder and the flaky aluminum powder is greater than or equal to 15 μm, and the diameter size deviation is less than or equal to 30%, the particle size of the titanium powder is 1.5 μm-5 μm, and the mass fraction of the titanium powder in the metal powder is 0.8%-15%; and

[0008] The zinc-aluminum-titanium protective liquid in the preform is dried and then sintered at a temperature of 250°C-370°C to form a protective layer on the surface of the sintered NdFeB magnet, wherein the protective layer includes zinc, aluminum, titanium, metal oxides containing zinc and metal oxides containing titanium.

[0009] In one embodiment, the mass fraction of the titanium powder in the metal powder is 0.8%-10%.

[0010] In one of the embodiments, in the metal powder, the weight ratio of the flaky zinc powder to the flaky aluminum powder is 3:1-7:1.

[0011] In one embodiment, the mass fraction of the metal powder in the zinc-aluminum-titanium protective liquid is 25%-35%, the mass fraction of the molybdate in the zinc-aluminum-titanium protective liquid is 2%-3%, and the mass fraction of the silane coupling agent in the zinc-aluminum-titanium protective liquid is 38%-45%.

[0012] In one embodiment, the average flake diameters of the flaky zinc powder and the flaky aluminum powder are independently selected from 15 μm to 100 μm.

[0013] In one embodiment, the molybdate is selected from ammonium molybdate.

[0014] The present invention increases the interlayer spacing of flaky zinc powder and flaky aluminum powder under a specific size by adding titanium powder to the metal powder raw material, and then effectively improves the dispersibility of the metal powder through the synergistic effect of titanium powder, flaky zinc powder and flaky aluminum powder, so that the metal powder is evenly distributed on the surface of the sintered NdFeB magnet. Furthermore, by sintering the zinc-aluminum-titanium protective liquid under specific temperature conditions, the metal powder can be partially oxidized under the oxidation of molybdate, which is conducive to promoting the cross-linking of the metal powder and the silane coupling agent, thereby forming an alternatingly stacked lamella network structure on the surface of the sintered NdFeB magnet, and effectively blocking the entry of corrosive media, thereby improving the protective performance of the protective layer. At the same time, titanium particles are distributed between the parallel stacked lamellae, which can form a conductive grid micro-region, which is conducive to dispersing the corrosion current and further improving the protective performance of the protective layer.

[0015] In addition, the protective layer and the sintered NdFeB magnet form a chemical bond through the silane coupling agent during the sintering process, which not only avoids the problem of corrosion of the sintered NdFeB magnet, allowing the sintered NdFeB magnet to maintain high magnetic properties, but also enhances the bonding force between the protective layer and the sintered NdFeB magnet.

[0016] A sintered NdFeB magnet coated with a protective layer is prepared by the surface protection method of the sintered NdFeB magnet as described above.

[0017] In one embodiment, the protective layer is a network structure of alternately stacked layers, wherein the layers are parallel to the surface of the sintered NdFeB magnet.

[0018] In one embodiment, the sheet network structure further includes titanium particles, and the titanium particles are distributed between the sheets.

[0019] In one embodiment, the thickness of the protective layer is 10 μm-1000 μm.

[0020] In the sintered NdFeB magnet coated with a protective layer described in the present invention, since the protective layer can play the role of sacrificial anode cathodic protection for the sintered NdFeB magnet, the sintered NdFeB magnet coated with the protective layer can significantly improve its corrosion resistance while maintaining the high magnetic properties of the sintered NdFeB magnet. Under the same conditions, the corrosion resistance of the sintered NdFeB magnet coated with the protective layer is improved by two orders of magnitude compared with the sintered NdFeB magnet, and the time it can withstand neutral salt spray can be as long as more than 1440 hours. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 The scanning electron microscope images of the flaky zinc powder, flaky aluminum powder and titanium powder raw materials used in Example 1, Example 2, Comparative Example 2 and Comparative Example 3 of the present invention are shown;

[0022] Figure 2The scanning electron microscope images of the surfaces of the sintered NdFeB magnets coated with the protective layer prepared in Example 1, Example 2, Comparative Example 5 and Comparative Example 6 of the present invention under different magnification conditions;

[0023] Figure 3 The surface scanning electron microscope image of the sintered NdFeB magnet coated with a protective layer prepared in Example 1 of the present invention and the element distribution map of zinc, aluminum and titanium in the surface;

[0024] Figure 4 The cross-sectional scanning electron microscope image and elemental analysis image of the sintered NdFeB magnet coated with a protective layer prepared in Example 1 of the present invention;

[0025] Figure 5 This is an X-ray diffraction pattern of the surface of the sintered NdFeB magnet coated with a protective layer prepared in Example 1 of the present invention;

[0026] Figure 6 This is an infrared spectrum of the surface of the sintered NdFeB magnet coated with a protective layer prepared in Example 1 of the present invention;

[0027] Figure 7 The thermogravimetric and differential thermal curves of the preform prepared in Example 1 of the present invention and the preform prepared in Comparative Example 1 during the heating process from 0 to 500° C. are shown;

[0028] Figure 8 The scanning electron microscope images of the magnet surface of the sintered NdFeB magnet coated with a protective layer prepared in Example 1 of the present invention after being tested for different test durations in a neutral salt spray test;

[0029] Fig. 9 The polarization curves of the sintered NdFeB magnet coated with a protective layer prepared in Example 1 of the present invention, the sintered NdFeB magnet coated with a protective layer prepared in Comparative Example 1, and the sintered NdFeB magnet in a 3.5% sodium chloride solution;

[0030] Fig.10 The polarization curves of the sintered NdFeB magnet coated with a protective layer prepared in Example 1 of the present invention in a 3.5% sodium chloride solution after different test durations in a neutral salt spray test;

[0031] Fig.11 The Nyquist diagram and Bode diagram of the sintered NdFeB magnet coated with a protective layer prepared in Example 1 of the present invention after being immersed in a 3.5% sodium chloride solution for different lengths of time, and the Nyquist diagram and Bode diagram of the sintered NdFeB magnet in a 3.5% sodium chloride solution after being tested for different lengths of time in a neutral salt spray test;

[0032] Fig.12Demagnetization curves and magnetic properties data analysis diagrams of the preform prepared in Example 1 of the present invention, the sintered NdFeB magnet coated with a protective layer prepared in Example 1, and the sintered NdFeB magnet coated with a protective layer prepared in Comparative Example 1. DETAILED DESCRIPTION

[0033] For ease of understanding of the present invention, the present invention will be described in more detail below. However, it should be understood that the present invention can be implemented in many different forms and is not limited to the embodiments or examples described herein. On the contrary, the purpose of providing these embodiments or examples is to make the understanding of the disclosure of the present invention more thorough and comprehensive.

[0034] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art of the present invention. The terms used in the specification of the present invention herein are only for the purpose of describing specific implementation methods or embodiments and are not intended to limit the present invention.

[0035] The present invention provides a surface protection method for a sintered NdFeB magnet, comprising the following steps:

[0036] S1, providing sintered NdFeB magnets;

[0037] S2, forming a zinc-aluminum-titanium protective liquid on the surface of the sintered NdFeB magnet to obtain a preform; wherein the zinc-aluminum-titanium protective liquid mainly comprises metal powder, molybdate and silane coupling agent, the metal powder comprises titanium powder, flaky zinc powder and flaky aluminum powder, wherein the average sheet thickness of the flaky zinc powder and the flaky aluminum powder are both less than or equal to 2.5 μm, and the thickness size deviation is less than or equal to 25%, the average sheet diameter of the flaky zinc powder and the flaky aluminum powder are both greater than or equal to 15 μm, and the diameter size deviation is less than or equal to 30%, the particle size of the titanium powder is 1.5 μm-5 μm, and the mass fraction of the titanium powder in the metal powder is 0.8%-15%; and

[0038] S3, drying the zinc-aluminum-titanium protective liquid in the preform and sintering it at a temperature of 250°C-370°C to form a protective layer on the surface of the sintered NdFeB magnet, wherein the protective layer includes zinc element, aluminum element, titanium element, metal oxide containing zinc and metal oxide containing titanium.

[0039] In order to enhance the bonding force between the surface of the sintered NdFeB magnet and the ZnAlTi protective liquid, in step S1, the sintered NdFeB magnet is preferably cleaned and dried to remove the oxide layer and other impurities on the surface of the sintered NdFeB magnet.

[0040] In step S2, titanium powder is added to the metal powder raw material to increase the interlayer spacing between flaky zinc powder and flaky aluminum powder at a specific size, and then the dispersibility of the metal powder is effectively improved through the synergistic effect of titanium powder, flaky zinc powder and flaky aluminum powder, so that the metal powder is evenly distributed on the surface of the sintered NdFeB magnet, which is conducive to obtaining a protective layer with high bonding strength and excellent protective performance.

[0041] After long-term and in-depth research, the applicant found that the content of titanium powder has an important influence on the performance of zinc-aluminum-titanium protective liquid. Titanium powder can not only increase the distance between flaky zinc powder and flaky aluminum powder, which is beneficial to improve the wettability of molybdate and silane coupling agent, but also improve the hardness of the protective layer, prevent the protective layer from producing through cracks, thereby improving the protective performance of the protective layer. However, when the amount of titanium powder added reaches a certain proportion, the protective layer after a certain temperature treatment will produce larger macro cracks, thereby greatly reducing the protective effect of the protective layer. Therefore, in order to make the protective layer have a higher protective effect and hardness, the mass fraction of the titanium powder in the metal powder is 0.8%-15%, and further, the mass fraction of the titanium powder in the metal powder is preferably 0.8%-10%, and more preferably 5%-10%.

[0042] At the same time, based on the chemical activity of aluminum, when the content of flaky aluminum powder in the zinc-aluminum-titanium protective liquid reaches a certain level, the protective effect of the protective layer also decreases with the increase in the specific gravity of the flaky aluminum powder in the zinc-aluminum-titanium protective liquid. Therefore, in order to make the protective layer not only have strong adhesion but also achieve long-term protective effect, in the zinc-aluminum-titanium protective liquid, the weight ratio of the flaky zinc powder to the flaky aluminum powder is 3:1-7:1, preferably 3:1-5:1.

[0043] In order to further improve the uniformity of the flaky zinc powder and the flaky aluminum powder on the surface of the sintered NdFeB magnet, preferably, the average lamella diameters of the flaky zinc powder and the flaky aluminum powder are independently selected from 15 μm to 100 μm.

[0044] In one embodiment, the mass fraction of the metal powder in the zinc-aluminum-titanium protective liquid is 25%-35%, the mass fraction of the molybdate in the zinc-aluminum-titanium protective liquid is 2%-3%, and the mass fraction of the silane coupling agent in the zinc-aluminum-titanium protective liquid is 38%-45%.

[0045] In one embodiment, the zinc-aluminum-titanium protective liquid further includes an emulsifier, a dispersant, a defoaming agent and an organic solvent.

[0046] Furthermore, the mass fraction of the emulsifier in the zinc-aluminum-titanium protective liquid is 9%-12%, the mass fraction of the dispersant in the zinc-aluminum-titanium protective liquid is 7%-10%, the mass fraction of the defoaming agent in the zinc-aluminum-titanium protective liquid is 1%-2%, and the mass fraction of the organic solvent in the zinc-aluminum-titanium protective liquid is 4%-7%.

[0047] Specifically, the emulsifier is selected from at least one of alkylphenol polyoxyethylene ether-10 (OP-10), polysorbate-20, and fatty alcohol polyoxyethylene ether (AE0-9); the dispersant is selected from at least one of polyethylene glycol-400 (PEG-400) and polyethylene glycol-200 (PEG-200); the silane coupling agent in the silane coupling agent hydrolyzate is selected from γ-(2,3-epoxypropoxy)propyltrimethoxysilane (KH-560); the defoamer is selected from polyether-modified silicone oil; and the organic solvent is selected from at least one of methanol and isooctyl alcohol.

[0048] In one embodiment, the organic solvent can be selected from a mixed solvent of methanol and isooctyl alcohol. Specifically, the weight fraction of methanol in the zinc-aluminum-titanium protective liquid is 3%-5%, and the weight fraction of isooctyl alcohol in the zinc-aluminum-titanium protective liquid is 1%-2%.

[0049] In order to promote the crosslinking of the metal powder and the silane coupling agent, the silane coupling agent is preferably mixed with deionized water at a weight ratio of 1:1.5-1:3 at 30°C-60°C, and then kept warm for 5h-20h to hydrolyze the silane coupling agent, and then mixed with the metal powder and molybdate.

[0050] In one embodiment, the zinc-aluminum-titanium protective liquid is formed on the surface of the sintered NdFeB magnet by spraying, dipping, spin coating or brushing, etc., which is not limited in the present application.

[0051] In step S3, the zinc-aluminum-titanium protective liquid in the preform is first dried, which is beneficial to the structural shaping of the protective layer and prevents the zinc-aluminum-titanium protective liquid from directly sintering at high temperature to cause macro cracks in the protective layer.

[0052] Furthermore, by sintering the zinc-aluminum-titanium protective liquid at 250°C-370°C, the metal powder can be partially oxidized under the oxidation of molybdate, which is conducive to promoting the cross-linking of the metal powder and the silane coupling agent, thereby forming an alternating stacked layer network structure on the surface of the sintered NdFeB magnet, and effectively blocking the entry of corrosive media, thereby improving the protective performance of the protective layer. At the same time, titanium particles are distributed between the parallel stacked layers, which can form a conductive grid micro-region, which is conducive to dispersing the corrosion current and further improving the protective performance of the protective layer.

[0053] In addition, the protective layer and the sintered NdFeB magnet form a chemical bond through the silane coupling agent during the sintering process, which not only avoids the problem of corrosion of the sintered NdFeB magnet, allowing the sintered NdFeB magnet to maintain high magnetic properties, but also enhances the bonding force between the protective layer and the sintered NdFeB magnet.

[0054] In one embodiment, the aluminum-containing metal oxide further includes molybdenum, and the molybdenum is derived from ammonium molybdate.

[0055] Specifically, the metal oxide containing zinc is selected from at least one of ZnO and ZnMoO4, and the metal oxide containing titanium is selected from TiO2.

[0056] In one embodiment, the protective layer further comprises a metal oxide containing aluminum.

[0057] In order to further analyze and explain the formation principle of the protective layer during the sintering process, the present invention provides the following chemical reaction formula (1):

[0058]

[0059] Among them, in the first stage, the silane coupling agent is hydrolyzed to generate Si-OH; in the second stage, Si-OH can condense with the hydroxyl groups (M1-OH, M1 represents zinc powder, aluminum powder, titanium powder) on the surface of titanium powder, flaky zinc powder, and flaky aluminum powder to form Si-O-M1, and can also condense with the hydroxyl groups (M2-OH, M2 represents sintered NdFeB magnet) on the surface of sintered NdFeB magnet to form Si-O-M2; in the third stage, Si-OH and Si-OH can form a low-polysiloxane Si-O-Si structure between Si-O-M1 and Si-O-M2 through dehydration condensation. Thus, a dense protective layer coated on the surface of the sintered NdFeB magnet can be formed after sintering.

[0060] Based on this, abundant chemical bonding is generated between the protective layer and the surface of the sintered NdFeB magnet due to sintering, which further enhances the bonding force between the protective layer and the sintered NdFeB magnet.

[0061] Specifically, the drying temperature is 80°C-100°C, and the time is 5min-15min; the sintering temperature is preferably 240°C-450°C, and the time is 10min-60min, preferably 25min-40min.

[0062] The invention also provides a sintered NdFeB magnet coated with a protective layer.

[0063] In the sintered NdFeB magnet coated with a protective layer described in the present invention, since the protective layer can play the role of sacrificial anode cathodic protection for the sintered NdFeB magnet, the sintered NdFeB magnet coated with the protective layer can significantly improve its corrosion resistance while maintaining the high magnetic properties of the sintered NdFeB magnet. Under the same conditions, the corrosion resistance of the sintered NdFeB magnet coated with the protective layer is improved by two orders of magnitude compared with the sintered NdFeB magnet, and the time it can withstand neutral salt spray can be as long as more than 1440 hours.

[0064] In one embodiment, the protective layer is a network structure of alternately stacked layers, wherein the layers are parallel to the surface of the sintered NdFeB magnet.

[0065] Furthermore, the sheet network structure also includes titanium particles, and the titanium particles are distributed between the sheets.

[0066] Specifically, the components of the titanium particles include titanium and titanium-containing metal oxides, wherein the titanium-containing metal oxides are selected from titanium dioxide.

[0067] In one embodiment, in a sintered NdFeB magnet coated with a protective layer, the thickness of the protective layer is 10 μm-1000 μm, which is beneficial to prevent micropores and microcracks on the surface of the protective layer from penetrating into the coating, thereby further improving the protective performance.

[0068] Hereinafter, the surface protection method of the sintered NdFeB magnet and the product thereof will be further described through the following specific examples.

[0069] Example 1

[0070] The sintered NdFeB magnets are cleaned and dried.

[0071] Flake zinc powder (diameter 19 μm, thickness 2 μm), flake aluminum powder (diameter 19 μm, thickness 2.5 μm) and titanium powder (particle size 3.3 μm) are used as metal powders, wherein the mass fraction of titanium powder in the metal powder is 10%, and the weight ratio of flake zinc powder to flake aluminum powder is 5:1. 33.5% of metal powder, 9.5% of OP-10, 8.9% of PEG-400, 4.67% of methanol, 2.8% of ammonium molybdate, 1.7% of isooctyl alcohol, 1.7% of polyether modified silicone oil and 42% of silane coupling agent hydrolyzate are mixed evenly to obtain zinc aluminum titanium protective liquid, wherein the silane coupling agent hydrolyzate is obtained by mixing KH-560 and deionized water in a weight ratio of 1:2 and then heat-insulating and hydrolyzing for 15 hours. The zinc aluminum titanium protective liquid is sprayed on the surface of the sintered NdFeB magnet to obtain a preform.

[0072] The preform was dried at 100° C. for 10 minutes and then sintered at 320° C. for 30 minutes to obtain a sintered NdFeB magnet coated with a protective layer.

[0073] The microscopic morphology of the raw materials of flaky zinc powder, flaky aluminum powder and titanium powder was analyzed by scanning electron microscope (SEM). Figure 1 As shown. Figure 1 It can be seen from (a) flaky zinc powder and (d) flaky aluminum powder that the flaky zinc powder and flaky aluminum powder with this diameter size tend to be distributed in parallel in a natural state without external force, while the particles of (c) titanium powder are irregular.

[0074] The surface of the sintered NdFeB magnet coated with a protective layer was analyzed by SEM at different magnifications. Figure 2 (b) and Figure 3 As shown in (a), it can be seen that there are some micropores and microcracks randomly distributed in the protective layer. Figure 3 Elemental analysis was performed in (a), and the results are as follows Figure 3 As shown in (b) to (d), it can be seen that the zinc, aluminum and titanium elements are evenly distributed in the protective layer without obvious agglomeration, and the titanium element is dispersed between the zinc and aluminum elements.

[0075] Furthermore, the cross-sectional morphology and elements of the sintered NdFeB magnet coated with the protective layer were analyzed. Figure 4 As shown. Figure 4 As can be seen in (a), the flaky zinc powder and flaky aluminum powder are stacked in parallel on the surface of the sintered NdFeB magnet, and hard titanium particles are wrapped between the flaky layers. The thickness of the protective layer is about 27μm. Figure 4 As shown in (b), part of the zinc element, a small amount of aluminum element and part of the titanium element in the protective layer diffused into the sintered NdFeB magnet.

[0076] The components of the surface of the sintered NdFeB magnet coated with the protective layer were analyzed by X-ray diffractometer (XRD). Figure 5 As shown, it can be seen that the protective layer mainly includes Zn, Al, Ti, TiO2, ZnO and ZnMoO4.

[0077] The surface of the sintered NdFeB magnet coated with the protective layer was analyzed by infrared spectrometer (FT-IR). Figure 6 As shown, it can be seen that at 3357cm -1 、1641cm -1 、858cm -1 and 906cm -1 The band observed at 2994 cm-1 belongs to the -OH vibration mode. -1 、2884cm -1 and 1257cm -1 The band observed at 2158 cm-1 and 1203cm -1 The bands observed at 1053 cm-1 belong to -Si-H and COC vibration modes. -1 and 1091cm -1 The spectral band observed at belongs to the Si-O-Si vibration mode. The appearance of the spectral band of the Si-O-Si vibration mode means that there is a slight condensation of the silane hydrolysis product.

[0078] Thermogravimetric analysis of the preform during the heating process from 0 to 500 °C was performed. Figure 7 As shown in (b), it can be seen that with the increase of temperature, the thermogravimetric curve shows obvious mass loss. Among them, the mass loss in the range of 30°C-97°C is related to the evaporation of volatile methanol in the solvent. Specifically, the endothermic peak at 92.3°C corresponds to the evaporation of methanol in the solvent. The mass loss between 109°C-155°C is due to the evaporation of water in the solvent and the condensation of hydroxyl groups connected to Si and C atoms. Specifically, the endothermic peak at 109.7°C in Figure (a) and the endothermic peak at 154.7°C in Figure (b) correspond to the volatilization of water molecules and the condensation of hydroxyl groups in the protective layer of Example 1 and the protective layer of Comparative Example 1, respectively, and according to these two endothermic peaks, it can be seen that the sintering temperature of the protective layer of Example 1 is higher than that of the protective layer of Comparative Example 1. The endothermic peak appearing at about 420°C indicates the decomposition of the organic functional groups of the silane coupling agent, which corresponds to the mass loss at the end of the curve. This also further proves the formation principle of the protective layer during sintering as shown in the chemical reaction formula (1).

[0079] Example 2

[0080] The sintered NdFeB magnets are cleaned and dried.

[0081] Flake zinc powder (diameter 19 μm, thickness 2 μm), flake aluminum powder (diameter 15 μm, thickness 2.5 μm) and titanium powder (particle size 3.3 μm) are used as metal powders, wherein the mass fraction of titanium powder in the metal powder is 5%, and the weight ratio of flake zinc powder to flake aluminum powder is 5:1. 33.5% of metal powder, 9% of OP-10, 8.9% of PEG-400, 4.67% of methanol, 2.8% of ammonium molybdate, 1.7% of isooctyl alcohol, 1.7% of polyether modified silicone oil and 42% of silane coupling agent hydrolyzate are mixed evenly to obtain zinc aluminum titanium protective liquid, wherein the silane coupling agent hydrolyzate is obtained by mixing KH-560 and deionized water in a weight ratio of 1:2 and then heat-insulating and hydrolyzing for 15 hours. The zinc aluminum titanium protective liquid is sprayed on the surface of the sintered NdFeB magnet to obtain a preform.

[0082] The preform is dried at 100°C for 10 minutes, and then sintered at 320°C for 30 minutes to obtain a sintered NdFeB magnet coated with a protective layer. The protective layer mainly includes Zn, Al, Ti, TiO2, ZnO and ZnMoO4.

[0083] Among them, the microscopic morphology of flake aluminum powder is as follows Figure 1 As shown in (e), the microscopic morphology of the surface of the sintered NdFeB magnet coated with the protective layer is as follows Figure 2 As shown in (a).

[0084] Example 3

[0085] The sintered NdFeB magnets are cleaned and dried.

[0086] Flake zinc powder (diameter 25 μm, thickness 2.5 μm), flake aluminum powder (diameter 18 μm, thickness 2 μm) and titanium powder (particle size 2 μm) are used as metal powders, wherein the mass fraction of titanium powder in the metal powder is 10%, and the weight ratio of flake zinc powder to flake aluminum powder is 3:1. 25% of metal powder, 9% of OP-10, 7% of PEG-400, 3% of methanol, 2% of ammonium molybdate, 1% of isooctyl alcohol, 1% of polyether modified silicone oil and 38% of silane coupling agent hydrolyzate are mixed evenly to obtain zinc aluminum titanium protective liquid, wherein the silane coupling agent hydrolyzate is obtained by mixing KH-560 and deionized water in a weight ratio of 1:2 and then heat-insulating and hydrolyzing for 15 hours. The zinc aluminum titanium protective liquid is sprayed on the surface of the sintered NdFeB magnet to obtain a preform.

[0087] The preform is dried at 100°C for 10 minutes, and then sintered at 300°C for 45 minutes to obtain a sintered NdFeB magnet coated with a protective layer. The protective layer mainly includes Zn, Al, Ti, TiO2, ZnO and ZnMoO4.

[0088] Example 4

[0089] The sintered NdFeB magnets are cleaned and dried.

[0090] Flake zinc powder (diameter 18μm, thickness 1.5μm), flaky aluminum powder (diameter 25μm, thickness 1.5μm) and titanium powder (particle size 5μm) are used as metal powders, wherein the mass fraction of titanium powder in the metal powder is 10%, and the weight ratio of flaky zinc powder to flaky aluminum powder is 7:1. 35% of metal powder, 12% of OP-10, 10% of PEG-400, 5% of methanol, 3% of ammonium molybdate, 2% of isooctyl alcohol, 2% of polyether modified silicone oil and 45% of silane coupling agent hydrolyzate are mixed evenly to obtain zinc-aluminum-titanium protective liquid, wherein the silane coupling agent hydrolyzate is obtained by mixing KH-560 and deionized water in a weight ratio of 1:2 and then heat-insulating and hydrolyzing for 15 hours. The zinc-aluminum-titanium protective liquid is sprayed on the surface of the sintered NdFeB magnet to obtain a preform.

[0091] The preform is dried at 100°C for 10 minutes, and then sintered at 280°C for 60 minutes to obtain a sintered NdFeB magnet coated with a protective layer. The protective layer mainly includes Zn, Al, Ti, TiO2, ZnO and ZnMoO4.

[0092] Comparative Example 1

[0093] The metal powder used was flaky zinc powder and flaky aluminum powder in a weight ratio of 5:1. The remaining preparation steps were the same as those in Example 1 to prepare a sintered NdFeB magnet coated with a protective layer.

[0094] Comparative Example 2

[0095] The difference between Comparative Example 2 and Example 1 is that the flaky zinc powder with a diameter of 10 μm and a thickness of 2 μm is used instead of the flaky zinc powder with a diameter of 19 μm and a thickness of 2 μm. Figure 1 (f) as shown.

[0096] Comparative Example 3

[0097] The difference between Comparative Example 3 and Example 1 is that the flaky aluminum powder with a diameter of 10 μm and a thickness of 2.5 μm is used instead of the flaky aluminum powder with a diameter of 19 μm and a thickness of 2.5 μm. Figure 1 (b) as shown.

[0098] Comparative Example 4

[0099] The difference between Comparative Example 4 and Example 1 is that the flaky zinc powder and the flaky aluminum powder in a weight ratio of 1:1 are used instead of the flaky zinc powder and the flaky aluminum powder in a weight ratio of 5:1.

[0100] Comparative Example 5

[0101] The difference between Comparative Example 5 and Example 1 is that 20% of titanium powder is used instead of 10% of titanium powder. The microscopic morphology of the surface of the sintered NdFeB magnet coated with the protective layer is shown in FIG. Figure 2 As shown in (c).

[0102] Comparative Example 6

[0103] The difference between Comparative Example 6 and Example 1 is that 40% of titanium powder is used instead of 10% of titanium powder. The microscopic morphology of the surface of the sintered NdFeB magnet coated with the protective layer is shown in FIG. Figure 2 As shown in (d).

[0104] The sintered NdFeB magnet coated with a protective layer prepared in Example 1 was subjected to a neutral salt spray test for different lengths of time. The surface morphology of the magnet after different test lengths is as follows: Figure 8 As shown. It can be seen that after 10 days of testing, cracks began to appear on the surface of the protective layer due to the increase of internal stress caused by corrosion; after 20 days of testing, the cracks on the surface of the protective layer were gradually covered or "repaired" by corrosion products; after 40 days of testing, the cracks on the surface of the protective layer basically disappeared, and the surface of the protective layer became denser; after 60 days of testing, the cracks on the surface of the protective layer increased, and the protective function of the protective layer began to fail.

[0105] Furthermore, element analysis was performed on the surface of the sintered NdFeB magnet coated with the protective layer after neutral salt spray tests of different lengths. The results are shown in Table 1.

[0106] Table 1

[0107]

[0108] Combined with Table 1, it can be further proved that after the test time reaches 60 days, the Fe content increases, indicating that the protective layer begins to fail.

[0109] The polarization curves of the sintered NdFeB magnet coated with the protective layer prepared in Example 1 and the sintered NdFeB magnet coated with the protective layer prepared in Comparative Example 1 were tested. The results are as follows: Fig. 9 And as shown in Table 2.

[0110] Table 2

[0111]

[0112] according to Fig. 9As can be seen from Table 2, compared with the sintered NdFeB magnet, the self-corrosion potential of the sintered NdFeB magnet coated with a protective layer in Example 1 and the sintered NdFeB magnet coated with a protective layer in Comparative Example 1 both have a significant negative shift, indicating that both protective layers can play a sacrificial anode cathodic protection role for the magnet. The self-corrosion potential of the sintered NdFeB magnet coated with a protective layer in Example 1 is 0.02V more negative than that of the sintered NdFeB magnet, and 0.21 more positive than that of the sintered NdFeB magnet coated with a protective layer in Comparative Example 1. According to the thermodynamic concept of corrosion potential, only the corrosion tendency can be explained. From the self-corrosion potential results, it can be seen that the corrosion resistance of the sintered NdFeB magnet coated with a protective layer in Example 1 is improved to a certain extent compared with the sintered NdFeB magnet coated with a protective layer in Comparative Example 1. The self-corrosion current density is a kinetic concept that can explain the corrosion rate of different samples. The self-corrosion current density of the sintered NdFeB magnet coated with a protective layer in Example 1 is one order of magnitude and two orders of magnitude lower than that of the sintered NdFeB magnet coated with a protective layer in Comparative Example 1 and the sintered NdFeB magnet, respectively. According to Faraday's law, for the same magnet, the corrosion rate is proportional to the corrosion current. Therefore, the corrosion resistance of the sintered NdFeB magnet coated with a protective layer in Example 1 is one order of magnitude higher than that of the sintered NdFeB magnet coated with a protective layer in Comparative Example 1, and is two orders of magnitude higher than that of the sintered NdFeB magnet. Compared with the sintered NdFeB magnet and the sintered NdFeB magnet coated with a protective layer in Comparative Example 1, the sintered NdFeB magnet coated with a protective layer in Example 1 has a higher polarization resistance and better corrosion resistance.

[0113] The sintered NdFeB magnet coated with a protective layer prepared in Example 1 was subjected to polarization curve tests in a 3.5% sodium chloride solution after different test durations in a neutral salt spray test. The results are as follows: Fig.10 And as shown in Table 3.

[0114] Table 3

[0115]

[0116] according to Fig.10As shown in Table 3, the self-corrosion potential of the sintered NdFeB magnet coated with the protective layer is similar to that of the sintered NdFeB magnet before 30 days of corrosion, the corrosion current density first increases and then decreases, and the polarization resistance (Rp) first decreases and then increases. This is because the electrochemically inert layer composed of oxides or silanes on the surface of the coating is destroyed by the electrolyte at the beginning of corrosion. The metal inside the protective layer is activated by corrosive ions, and as the area of ​​metal zinc and aluminum activated gradually increases, the corrosion current density gradually increases. This shows that the protective layer is more susceptible to corrosion as an anode, and the protection mechanism of the protective layer is to cathodically protect the NdFeB magnet by sacrificing zinc and aluminum. As the corrosion time reaches 20 days, Rp gradually increases, and the corrosion current density increases by an order of magnitude compared with corrosion for 10 days, indicating that the corrosion products produced by the metal zinc, aluminum and their oxides in the protective layer are precipitated in the micropores and microcracks of the protective layer, hindering the penetration of the electrolyte and reducing the active area of ​​the metal sheet. When the corrosion time reaches 30 days, the surface of the protective layer becomes denser, the corrosion current density is reduced by an order of magnitude compared with corrosion for 20 days, and the corrosion rate slows down. As the corrosion time extends to 40 days, the self-corrosion potential gradually shifts positively, the corrosion current density gradually decreases, the polarization resistance increases, the corrosion tendency of the protective layer weakens, and the corrosion rate slows down. After the protection time of the protective layer reaches 60 days, the corrosion current density is one order of magnitude larger than the magnet, and the polarization resistance decreases, which indicates that the corrosive medium directly contacts the NdFeB magnet through the cracks or holes after the corrosion of the protective layer, and the protective function of the protective layer begins to fail.

[0117] The sintered NdFeB magnets coated with a protective layer prepared in Example 1 were immersed in a 3.5% sodium chloride solution for different lengths of time and tested in a neutral salt spray test for different lengths of time. The performance tests were performed on the samples. The results are as follows: Fig.11 shown.

[0118] The magnetic properties of the sintered NdFeB magnet coated with a protective layer prepared in Example 1, the sintered NdFeB magnet coated with a protective layer prepared in Comparative Example 1, and the preform prepared in Example 1 were tested. The results are as follows: Fig.12 As shown. Fig.12 (a) It can be seen that the demagnetization curves of the preform of Example 1, the sintered NdFeB magnet coated with a protective layer of Example 1, and the sintered NdFeB magnet coated with a protective layer of Comparative Example 1 are almost identical, indicating that the protective layer has almost no significant effect on the inherent magnetic properties of the sintered NdFeB magnet. Fig.12 (b) It can be seen that compared with the preform of Example 1, the coercive force (Hcj), remanence (Br) and maximum magnetic energy product ((BH)max) of the sintered NdFeB magnet coated with a protective layer in Example 1 have almost no changes, while the squareness (Squareness) of the magnet is slightly reduced, which means that the sintering treatment has basically no change on the magnetic properties of the preform, but the ability to resist the external magnetic field is slightly reduced.

[0119] The comprehensive performance tests of Examples 1-4 and Comparative Examples 4-6 were performed, and the results are shown in Table 4.

[0120] Table 4

[0121]

[0122] According to Examples 1-4 in Table 4, it can be seen that when the ratio of flaky zinc powder to flaky aluminum powder in the coating is 7:1 or 5:1, the dispersibility of the zinc-aluminum-titanium protective liquid is better; when the ratio of flaky zinc powder to flaky aluminum powder is 5:1, the hardness of the protective layer is better; when the ratio of flaky zinc powder to flaky aluminum powder is 3:1, the adhesion of the protective layer is better; when the ratio of flaky zinc powder to flaky aluminum powder is between 7:1 and 3:1, the corrosion resistance of the sintered NdFeB magnet coated with the protective layer is better. According to Comparative Example 4, when the ratio of flaky zinc powder to flaky aluminum powder is 1:1, the corrosion resistance is reduced, which may be because the aluminum powder is more active and the corrosion rate in the corrosive liquid is too fast to play a long-term protective role. Therefore, considering the various properties of the coating and the sintered NdFeB magnet coated with the protective layer, when the ratio of zinc-aluminum-titanium is 5:1, the comprehensive performance of the sintered NdFeB magnet coated with the protective layer is the best.

[0123] It can be seen from Examples 1-4 and Comparative Examples 4-6 in Table 4 that with the increase of titanium content, the corrosion resistance and hardness of the protective layer first increase and then decrease. The protective layer with a titanium powder content of 10% in the metal powder has better performance. When the titanium powder content exceeds 15%, larger macro cracks will appear, thereby reducing the performance of the protective layer.

[0124] The technical features of the above-described embodiments may be arbitrarily combined. To make the description concise, not all possible combinations of the technical features in the above-described embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0125] The above-mentioned embodiments only express several implementation methods of the present invention, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention patent. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the attached claims.

Claims

1. A surface protection method for sintered NdFeB magnets, It is characterized in that The following steps are involved: Provide sintered NdFeB magnets; A zinc-aluminum-titanium protective liquid is formed on the surface of the sintered NdFeB magnet to obtain a preform; wherein the zinc-aluminum-titanium protective liquid comprises metal powder, molybdate and silane coupling agent, the mass fraction of the metal powder in the zinc-aluminum-titanium protective liquid is 25%-35%, the mass fraction of the molybdate in the zinc-aluminum-titanium protective liquid is 2%-3%, and the mass fraction of the silane coupling agent in the zinc-aluminum-titanium protective liquid is 38%-45%; The metal powder comprises titanium powder, flaky zinc powder and flaky aluminum powder, wherein the average flaky zinc powder and the flaky aluminum powder have an average flaky layer thickness less than or equal to 2.5 μm, and a thickness size deviation less than or equal to 25%, the average flaky layer diameter of the flaky zinc powder is 18 μm-25 μm, the average flaky layer diameter of the flaky aluminum powder is 15 μm-25 μm, and the diameter size deviation of the flaky zinc powder and the flaky aluminum powder is less than or equal to 30%, the weight ratio of the flaky zinc powder to the flaky aluminum powder is 3:1-7:1, the particle size of the titanium powder is 2 μm-5 μm, and the mass fraction of the titanium powder in the metal powder is 5%-10%; and The zinc-aluminum-titanium protective liquid in the preform is dried and then sintered at a temperature of 250°C-370°C to form a protective layer on the surface of the sintered NdFeB magnet, wherein the protective layer includes zinc, aluminum, titanium, metal oxides containing zinc and metal oxides containing titanium.

2. The surface protection method of the sintered NdFeB magnet according to claim 1, It is characterized in that The mass fraction of the titanium powder in the metal powder is 5% or 10%.

3. The surface protection method of the sintered NdFeB magnet according to claim 1, It is characterized in that The average flake diameter of the flaky zinc powder is 19 μm-25 μm; And / or, the average flake diameter of the flaky aluminum powder is 18 μm-25 μm.

4. The surface protection method of sintered NdFeB magnet according to claim 1, It is characterized in that The molybdate is selected from ammonium molybdate.

5. A sintered NdFeB magnet coated with a protective layer, It is characterized in that The sintered NdFeB magnet coated with the protective layer is prepared by the surface protection method of the sintered NdFeB magnet as claimed in any one of claims 1 to 4.

6. The sintered NdFeB magnet coated with a protective layer according to claim 5, It is characterized in that The protective layer is a stacked layer network structure, wherein the layers are parallel to the surface of the sintered NdFeB magnet.

7. The sintered NdFeB magnet coated with a protective layer according to claim 6, It is characterized in that The sheet network structure also includes titanium particles, which are distributed between the sheets.

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